Disengagement of motor cortex from movement control during long-term learning

Abstract

Motor learning involves reorganization of the primary motor cortex (M1). However, it remains unclear how the involvement of M1 in movement control changes during long-term learning. To address this, we trained mice in a forelimb-based motor task over months and performed optogenetic inactivation and two-photon calcium imaging in M1 during the long-term training. We found that M1 inactivation impaired the forelimb movements in the early and middle stages, but not in the late stage, indicating that the movements that initially required M1 became independent of M1. As previously shown, M1 population activity became more consistent across trials from the early to middle stage while task performance rapidly improved. However, from the middle to late stage, M1 population activity became again variable despite consistent expert behaviors. This later decline in activity consistency suggests dissociation between M1 and movements. These findings suggest that long-term motor learning can disengage M1 from movement control.

Fig. 1. Task performance and movement consistency improve over long-term training in the joystick task.

(A) The joystick task setup. The mouse is required to move the joystick into the target upon the auditory go cue to receive a water reward. (B) The success rate (i.e., fraction of trials that acquired reward over all trials; left) and time required to acquire reward (right) as a function of training day. Mean ± SEM (n = 12 mice). P values are from two-sided pairwise comparison between the average values in the early (days 1 to 3) and late learning stages (days 50 to 60). (C) Variability of movement onset time (left) and movement duration (right) as a function of training day. SDs were used to measure variability. (D) Joystick movement trajectories from three different training days of a single mouse. Thirty trials in each condition are shown. (E) Trial-to-trial movement trajectory correlation as a function of training day. (F) Trial-to-trial trajectory correlation between two training days, averaged across 12 mice. The diagonal squares represent the trial-to-trial correlation within single training days plotted in (E).

Fig. 2. M1 inactivation effects on movements gradually decline during long-term training.

(A) Inactivation/head-bar control experiments in the early (days 1 to 9), mid (days 19 to 26), or late learning stage (days 61 to 69). (B) M1 inactivation and head-bar control days were randomly interleaved in each learning stage. The blue LED light was turned on in randomly selected trials (~12%) in each day. These light-on trials in the inactivation and head-bar days are referred to as inactivation and control trials, respectively, in the following comparisons. (C) The success rate and the fraction of trials in which mice made no movements out of all trials, in control versus inactivation trials, at the early, middle, and late learning stage. Thin lines represent individual mice, and thick lines represent medians across mice. Two-sided pairwise comparison between control and inactivation trials within each learning stage are displayed. For the effect size comparison between different learning stages, one-sided unpaired comparison was performed on the differences between control and inactivation trials in each stage. n.s., not significant. (D) Inactivation-induced changes (inactivation − control) in the success rate and the fraction of no movements out of all trials. The circles represent individual mice. The edges of the boxes mark the 25th and 75th percentiles, the whiskers extend to the most extreme nonoutlier data points, and the red lines indicate the medians across mice. The same statistical tests as in (C) are displayed. (E) Inactivation effects on trials in which mice initiated movements. The fraction of trials that mice initiated a movement but failed to reach the target, movement onset time, and peak velocity in control versus inactivation trials at the three learning stages. (F) Inactivation-induced changes in the fraction of failure, movement onset time, and peak velocity in all initiated movements.

Fig. 3. M1 inactivation affects successful movements and grips on the joystick at the early stage.

(A) Inactivation effects on movements that successfully entered the target. The peak velocity, path length, number of attempts to reach the target, and movement duration, in control versus inactivation trials, at the three learning stages. Mice that did not make any successful movement under inactivation were excluded from this analysis. Two-sided pairwise comparison between control and inactivation trials within each learning stage are displayed. For the effect size comparison between different learning stages, one-sided unpaired comparison was performed on the differences between control and inactivation trials in each stage. (B) Inactivation-induced changes in the peak velocity, path length, number of attempts, and movement duration of all movements that successfully entered the target. The same statistical tests as in (A) are displayed. (C) Example frames from videography showing the forepaw (red arrowhead) and the joystick (blue arrowhead) during a control trial (top) and an inactivation trial (bottom) during the early stage of learning. (D) The fraction of trials in which mice lost their grip on the joystick, in control versus inactivation trials, at the three learning stages. (E) Inactivation-induced changes in the fraction of trials in which mice lost their grip on the joystick.

Fig. 4. M1 population activity consistency evolves in two phases over long-term training.

(A) Longitudinal imaging of the neurons in the same field in M1 over the course of 60-day training in the joystick task. (B) Task performance and movement consistency during the 60-day training in the imaging mice: the success rate, time required to acquire reward, variability of movement onset time, variability of movement duration, and trial-by-trial movement trajectory correlation as a function of training day, from left to right. Mean ± SEM (n = 5 mice). The three shaded regions correspond to days 1 to 3, 19 to 29, and 50 to 60, respectively. Two-sided pairwise comparison for each pair of learning stages is displayed. (C) The imaging fields from training days 3, 21, 40, and 60 of a single mouse. (D) The SNR and the number of neurons in the imaging field as a function of training day. Mean ± SEM (n = 5 mice). The blue, green, and red shaded regions correspond to the early, middle, and late learning stages in the inactivation experiments, respectively. (E) Movement trajectory, four single-neuron activity, and whole-population activity (heat map) from four example trials in a single day. The vertical lines mark movement onset. The trajectory and single neuron activity of trial 1 were superposed in the other trials as a thin red line for comparison. The neurons in the heat map are sorted in the same way in all four trials according to the peak activity time in trial 1. The three numbers above the heat maps indicate the trial-to-trial population activity correlation for the corresponding trial pairs. (F) The trial-to-trial population activity correlation as a function of training day (bin size: 3 days). Thin different colors represent different mice, and the black line represents mean ± SEM (n = 5 mice). The three shaded regions include bins corresponding to the three learning stages in (D). Two-sided pairwise comparison for each pair of learning stages is displayed.

Fig. 5. The relationship between M1 activity and movements varies with learning stages.

(A) The relationship between trial-to-trial movement trajectory correlation and population activity correlation over the 60-day training (bin size: 3 days). The x axis represents the movement correlation binned in nine intervals, and the y axis represents the mean population activity in each interval. Thin different colors represent different mice, and the black line represents mean ± SEM (n = 5 mice). (B) The evolution of population activity consistency over 60 days for movement pairs with low, middle, and high movement trajectory correlations, from left to right. The three shaded regions are the same as the three learning stages in Fig. 4F. The increase followed by decrease of activity correlation is observed consistently in all groups of movement correlations. (C) A model for the evolution of M1 engagement over the course of long-term motor learning. The thickness of the lines between neural activity patterns and movement patterns indicate the degree of M1 dependence of movements inferred from the inactivation experiments. The consistency and degeneracy between activity and movement patterns are derived from the imaging experiments.